[3.0][Testing] Anchor the canonical path test to the root the platform actually uses - #9600

Description

SapiTest::testCanonicalPathResolvesDotSegments() asserted a POSIX-only result, so it failed on Windows with C:\a\c where it expected /a/c.

The production code is right. On Windows, canonicalPath('/a/./b/../c') normalises the separators and then falls into the "Windows relative path" branch: a path that starts at the root but names no drive is rooted on the current drive, so getcwd()'s drive letter is prepended. C:\a\c is the correct canonical form of \a\c there, and realpath() would say the same thing. The test simply baked in the shape an absolute path has on one platform.

Naming the root in the path under test takes the DOS-style-path branch on Windows instead, so the result no longer depends on which drive the checkout happens to sit on:

$root = DIRECTORY_SEPARATOR === '/' ? '' : 'C:';
$sep = DIRECTORY_SEPARATOR;

$this->assertSame($root . $sep . 'a' . $sep . 'c', Sapi::canonicalPath($root . '/a/./b/../c', false, false));
$this->assertSame($root . $sep . 'a', Sapi::canonicalPath($root . '/a/b/..', false, false));

The assertion is then about the dot segments, which is what the test is named for, rather than about the machine running it.

Verified by extracting the function body, substituting \ for DIRECTORY_SEPARATOR and running it: the old assertions produce C:\a\c and C:\a exactly as reported, and the new ones hold. The suite is green on Linux, and composer lint is clean.

The rest of tests/Unit/ has no other case of this. The remaining '/...' literals are URL routes rather than file paths, and LangTest compares canonicalPath() output against canonicalPath() output, so both sides move together.

Issues References (Fixes|Related|Closes)

1. Fixes SapiTest fails on Windows #9595